In the middle of all the blithering stupidity generated by conservatives about what's going on in Russia/Ukraine/Crimea (and apparently, it's no long "the" Crimea, just Crimea), I'd like to post the text of the 1994 Budapest Memorandum on Security Assurances. This treaty (non-ratified by the Senate, so technically not even a treaty) does not commit the U.S. or the U.K. to intervene militarily in case Ukraine's sovereignty is threatened. Article 4 is the key part - it applies in the case of an attack on the Ukraine involving nuclear weapons as the trigger for signatory action.

Taking into account the commitment of Ukraine to eliminate all
nuclear weapons from its territory within a specified period of time,

Noting the changes in the world-wide security situation, including
the end of the Cold War, which have brought about conditions for deep
reductions in nuclear forces.

Confirm the following:

The United States of America, the Russian Federation, and the United
Kingdom of Great Britain and Northern Ireland, reaffirm their
commitment to Ukraine, in accordance with the principles of the CSCE Final Act, to respect the Independence and Sovereignty and the existing borders of Ukraine.

The United States of America, the Russian Federation, and the United
Kingdom of Great Britain and Northern Ireland, reaffirm their
obligation to refrain from the threat or use of force against the
territorial integrity or political independence of Ukraine, and that
none of their weapons will ever be used against Ukraine except in
self-defense or otherwise in accordance with the Charter of the United Nations.

The United States of America, the Russian Federation, and the United
Kingdom of Great Britain and Northern Ireland, reaffirm their
commitment to Ukraine, in accordance with the principles of the CSCE
Final Act, to refrain from economic coercion designed to subordinate to
their own interest the exercise by Ukraine of the rights inherent in its
sovereignty and thus to secure advantages of any kind.

The United States of America, the Russian Federation, and the United
Kingdom of Great Britain and Northern Ireland, reaffirm their
commitment to seek immediate United Nations Security Council action to
provide assistance to Ukraine, as a non-nuclear-weapon State Party to
the Treaty on the Non-Proliferation of Nuclear Weapons, if Ukraine
should become a victim of an act of aggression or an object of a threat
of aggression in which nuclear weapons are used.

The United States of America, the Russian Federation, and the United
Kingdom of Great Britain and Northern Ireland, reaffirm, in the case of
the Ukraine, their commitment not to use nuclear weapons against any
non-nuclear-weapon State Party to the Treaty on the Non-Proliferation of
Nuclear Weapons, except in the case of an attack on themselves, their
territories or dependent territories, their armed forces, or their
allies, by such a state in association or alliance with a nuclear weapon
state.

The United States of America, the Russian Federation, and the United
Kingdom of Great Britain and Northern Ireland will consult in the event
a situation arises which raises a question concerning these
commitments.

This Memorandum will become applicable upon signature.

Signed in four copies having equal validity in the English, Russian and Ukrainian languages.